fix(@nestjs/swagger): replace quoted functionPlaceholder completely

[Panzer1119]

When building JS InitOptions, functions get replaced by a placeholder.
JSON.stringify quotes strings, so we need to replace the placeholder
including the quotes.

Fixes: #1962

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #1962

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[flamewow]

LGTM
one minor comment +
@Panzer1119 can you please add a function to e2e/manual-e2e.ts to cover the case you've found

smth like:

  SwaggerModule.setup('/swagger-docs', app, document, {
    customSiteTitle: 'Demo API - Swagger UI 2',
    swaggerOptions: {
      persistAuthorization: true,
      defaultModelsExpandDepth: -1,
      operationsSorter: (a, b) => a.get('id').localeCompare(b.get('id')),
    }
  });

[Panzer1119]

Done

[kamilmysliwiec]

LGTM